This invention relates generally to deflection control systems and specifically to a temperature stabilized horizontal deflection control system.
Deflection control systems for cathode ray tube (CRT) displays are well known in the art. It is quite common in color television receivers for example to use horizontal deflection control systems that are of the so-called closed loop type. Generally, a phase-locked or voltage controlled oscillator is used to maintain the phase and frequency of the horizontal deflection system within precisely defined limits. In computer monitor applications and in small screen monochrome television receivers, open loop type deflection control systems are used because of their simplicity and low cost and because the deflection drive requirements in small screen applications are much less exacting. The lower drive results in a cooler operating base-emitter junction for the horizontal output transistor. The result is that few if any problems are encountered from delays in switching, at least for operation under normal ambient temperature conditions. In certain applications, however, there is a special need for very close control of the position of the display on the CRT face. This is true with computer display terminals incorporating touch-activated CRT display panels. In such systems very close tolerances must be maintained on the horizontal deflection system to insure that the display is properly positioned on the CRT screen in correct alignment with the touch panel.
Also, for CRT displays intended for vehicular use in environments experiencing a very wide range of temperatures, simple low cost, open loop deflection systems may not be satisfactory. Specifically, the horizontal output transistor delay may be several microseconds corresponding to a base-emitter junction voltage change of as much as 0.2 volts over the approximately 100.degree. C. temperature range encountered. As the base-emitter junction temperature rises, transistor switching becomes slower or more sluggish. Thus, an uncorrectable switching delay, responsive to the horizontal output transistor base-emitter junction temperature, occurs with the result that the display is horizontally displaced on the CRT screen. This is an undesirable condition.